What blood tests are done to test the thyroid?
Thyroid blood tests include:
- Thyroid-stimulating hormone (TSH) is produced in the pituitary gland and regulates the balance of thyroid hormones –- including T4 and T3 -- in the bloodstream. This is usually the first test your provider will do to check for thyroid hormone imbalance. Most of the time, thyroid hormone deficiency (hypothyroidism) is associated with an elevated TSH level, while thyroid hormone excess (hyperthyroidism) is associated with a low TSH level. If TSH is abnormal, measurement of thyroid hormones directly, including thyroxine (T4) and triiodothyronine (T3) may be done to further evaluate the problem. Normal test range for an adult: 0.40 - 4.50 mIU/mL (milli-international units per liter of blood).
- T4: thyroxine tests for hypothyroidism and hyperthyroidism, and used to monitor treatment of thyroid disorders. Low T4 is seen with hypothyroidism, whereas high T4 levels may indicate hyperthyroidism. Normal range for an adult: 5.0 – 11.0 ug/dL (micrograms per deciliter of blood).
- FT4: Free T4 or free thyroxin is a method of measuring T4 that eliminates the effect of proteins that naturally bind T4 and may prevent accurate measurement. Normal test range for an adult: 0.9 - 1.7 ng/dL (nanograms per deciliter of blood)
- T3: triiodothyronine tests help diagnose hyperthyroidism or to show the severity of hyperthyroidism. Low T3 levels can be observed in hypothyroidism, but more often this test is useful in the diagnosis and management of hyperthyroidism, where T3 levels are elevated. Normal range: 100 - 200 ng/dL (nanograms per deciliter of blood).
- FT3: Free T3 or free triiodothyronine is a method of measuring T3 that eliminates the effect of proteins that naturally bind T3 and may prevent accurate measurement. Normal range: 2.3 - 4.1 pg/mL (picograms per milliliter of blood)
These tests alone aren’t meant to diagnose any illness but may prompt your healthcare provider to do additional testing to evaluate for a possible thyroid disorder.
Additional blood tests might include:
- Thyroid antibodies: These tests help identify different types of autoimmune thyroid conditions. Common thyroid antibody tests include microsomal antibodies (also known as thyroid peroxidase antibodies or TPO antibodies), thyroglobulin antibodies (also known as TG antibodies), and thyroid receptor antibodies (includes thyroid stimulating immunoglobulins [TSI] and thyroid blocking immunoglobulins [TBI]).
- Calcitonin: This test is used to diagnose C-cell hyperplasia and medullary thyroid cancer, both of which are rare thyroid disorders.
- Thyroglobulin: This test is used to diagnose thyroiditis (thyroid inflammation) and to monitor treatment of thyroid cancer.